The present invention pertains to the conversion of analog signals to digital format. It is particularly applicable for capturing wideband signals where the desired dynamic range of the system cannot be achieved by existing analog to digital converters. There are many applications where the system is limited by the dynamic range of the digitizer. Some of the applications with this limitation include specific emitter identification (SEI) systems, high frequency test equipment, and CDMA cell towers.
The conversion speed and dynamic range of conventional digitizers is limited by several factors. An article authored by R. H. Walden, and entitled Analog-to-Digital Converter Survey and Analysis, published in IEEE Communications, Vol. 17, No. 4, pp. 539-550, April 1999, aptly describes these factors and shows that as the sampling rate increases, the dynamic range degrades. In particular, the dynamic range decreases by one effective bit (6.02 dB) for each octave increase in instantaneous bandwidth. There has been significant effort put toward finding a solution around this digitizer limitation. Most of the solutions involve time interleaving multiple digitizers together so that they each sample one after the other. This allows for a sampling rate beyond the performance of only one digitizer, but it does not get around the problem caused by the time aperture window of each sample. According to the aforementioned article, this time aperture window, known as jitter, is the primary limitation to the dynamic range of a digitizer.
One way to get around the jitter limitation is to filter the signal into frequency slices or channels and then mix the frequency channels down to baseband where they are digitized by slower digitizers. This type of channelized design can get around the jitter limitation because each digitizer is sampling a much narrower band signal at a lower frequency. The channelized system is dependent on the stability of the local oscillators for the mixers rather than the digitizer. The stability of a frequency oscillator can be much greater than the jitter of a digitizer, so combining the channels is, in effect, multiplying the performance of the digitizers used to sample the channels.
Channelized systems have been attempted for many years, but have not been successful for several reasons. First and foremost is the difficulty of coherently recombining the channels together. Most RF filters cause significant distortion to the signal information at the band edges. Coherently combining the band edges of adjacent filters has proved to be an impossible task prior to this invention. The second difficulty involves aligning the channels in time and knowing the phase of each local oscillator during the signal acquisition. The final difficulty is removing the amplitude and phase distortions that are introduced by the RF and electronic elements in the system. These three difficulties combined have made the realization of a channelized system impossible prior to this invention.